- Email: [email protected]
Influence of additives to chromium oxide catalysts for the thermal decomposition of KClO4
Surface Technology, 21 (1984) 117 - 123
117
INFLUENCE OF ADDITIVES TO CHROMIUM OXIDE CATALYSTS FOR THE THERMAL DECOMPOSITION OF KClO4
E. A. HASSAN, A. A. SAID and K. M. ABD EL-SALAAM Chemistry Department, Assiut University, Assiut (Egypt)
(Received June 22, 1983)
Summary The effect of doping on the catalytic behaviour of Cr203 was studied using the thermal decomposition of KCIO4 as a test reaction. The doping of the catalysts was effected using 5 mol.% Li÷, Zr 4÷ or U 6÷ ions and calcination at 600 °C for 5 h. Studies were carried out using thermogravimetric analysis and electrical conductivity measurements. The results showed that Cr203 was more effective in enhancing the decomposition process when it was doped with U 6÷ ions than when it was doped with Li÷ or Zr 4÷ ions. Mechanisms of doping were discussed and the role of defects in the catalytic reaction was interpreted.
1. Introduction Several interesting discussions have concerned the relationship between the electronic structure o f solids and their catalytic activity [1, 2]. Several researchers have been concerned with the surface structure of solid chromium oxide with particular reference to the identification of the active sites responsible for the catalytic activity [3 - 6 ] . Fischer and Dietrich [7] studied the electrical behaviour o f Cr203 and its dependence on the temperature and partial pressure of oxygen. It was found that Cr203 was an electron~leficient conducting solid. This result was in accordance with that of Tavadzo et al. [8], who indicated that Cr203 is a p-type semiconductor and at high temperatures the structure becomes disordered, possibly as a result of Frenkel defect formation. Incorporation of CdO into Cr203 was examined [9] at 873 K and it was found using the electronic conduction m e t h o d that oxygen vacancies are the predominant defect. Addition of a monovalent cation to Cr203 showed an increase in the conductivity and had a great influence on its activity [10]. The activity of Cr203 as a dehydrogenation catalyst for 2-propanol was examined where a reversible p - n transition was observed [11]. The dehydrogenation dominated with increasing n-type character and increasing conductivity. 0376~4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
118 According to work reported in the literature and our previous studies [12] it is o f interest to study the effect o f additives such as Li +, Zr 4÷ or U 6÷ ions to the Cr203 lattice structure on t he catalytic action o f Cr203 for the thermal d eco mp o s it i on o f KC104 as a test reaction.
2. Experimental details Chromium oxide catalysts d o p e d with 5 mol.% Li ÷, Zr 4+ or U 6÷ ions were prepared by mixing calculated amounts o f LiOH, Zr(NO3)3.3H20 or UO2(NO3)2.6H20 and Cr(NO3)3.3H20 in a porcelain dish using d o u b l y distilled water. The mixtures were stirred well, evaporated over a water bath to dryness and then dried in an oven at 110 °C to constant weight. The catalysts were calcined in a muffle furnace in an air stream at 600 °C for 5 h. The thermal decomposition reaction o f AnalaR KC104 was used for monitoring the doping effect on the catalytic behaviour o f Cr203. These studies were carried o u t using a gasometric technique [ 13 ]. The electrical conductivity measurements were carried o u t as previously discussed [14]. IR spectra were taken for the final d e c o m p o s i t i o n products using a P e r k i n - E l m e r s pe c t r om e t e r model 599B with t he KBr disc technique. The oxidation powers o f t h e catalysts were determined using t he KI m e t h o d [15].
3. Results and discussion Thermograms o f pure and catalysed KC104 decom posi t i on are shown in Fig. 1. Curve a represents the thermal de c omposi t i on o f pure KC104 at a pressure o f 1 atm. The decomposition o f KC104 catalysed with 10% Cr203 is represented by curves b - e. The general t r e nd is for the catalyst to cause an e n h a n c e m e n t in the decom pos i t i on reaction to a certain degree, depending on t h e catalyst preparation. Also, it appears t h at the most effective doping o f the Cr203 catalyst is by U 6÷ ions. The exact differences in the decomposition temperatures can be seen when differential thermogravimetric plots are constructed as shown in Fig. 2. A s um m a r y o f the results is given in Table 1. Such a great difference in t he Td values m a y be understood by studying t h e mechanism o f doping o f t he Cr203 structure. Figure 3 shows t he variation in log a against 1/T f or pure and doped Cr203 catalysts. The results show th at when Cr203 is doped with U 6+ ions it has a smaller conductivity value at any t e m p e r a t u r e than when it is d o p e d with Li ÷ or Zr 4+. It is well k n o w n that Cr203 is a p-type s e m i conduct or [8]; its c o n d u c t a n c e charge carriers are holes according to 3 1 - - 0 2 = [Cr]"' + 3[el" + --Cr203 4 2
(1)
where [Cr[ '" is a cation vacancy and [el" is a defect electron. The addition of
119
9ok to
O-
E
70
123
50
3O
~0
200
300
L.O0
500 Temp.°C
Fig. 1. Thermogravimetrie analysis of the thermal decomposition of KCIO4 (curve a) and of KCIO4 catalysed with pure 10% Cr203 (curve b) and doped with 5 tool.% Li+ (curve c), 5 tool.% Zr4+ (curve d) and 5 tool.% U6+ (curve e).
U 6+ ions consequently causes a decrease in the n u m b e r o f these charge carriers according to the following mechanism: 2Jel" + 2UO2 = 2UICri" + Cr203 + iOl'"
(2)
6[el" + 2UO3 = 2 U [ C r [ " " + Cr203 + 3101""
(3)
where U 6+ ions exist in the oxide as t he U3Os structure [16] and diffuse into t h e Cr203 solid lattice according to mechanisms (2) and (3). UHCr[" and U [ C r I " " represent U 4+ and U 6+ which replace the Cr 3÷ in their normal lattice site positions and IO]'" is an o x y g e n anion vacancy. According to these mechanisms a decrease in the ie[" concent rat i on is predicted, which eventually leads to a r e duc t i on in o as observed in Fig. 3. Adopting such a role for the doping effect, t hen Zr 4÷ ions can be i ncorporat ed in the following way: 21el" + 2ZrO2 = 2ZrlCri" + Cr203 + ]O['"
(4)
Such a mechanism will lead to a decrease in the hole c o n c e n t r a t i o n and consequently to a reduction in a compared with t he conductivity o f pure Cr20 3 which is n o t f ound experimentally. Also, with Li + ions, doping could be effected in the following way:
120
I
1e
I
I
t
11 II
II
iI
d I
, I
b
~1 / .~Ii i~,ll
,
o
I, !t,
IL i,!, II
l
I' , 300
\
: ;i,/I I
[I
I!Jl
I
~ill
I
/
~
/
' i'a I
/
/* .ff /.00
500
600
Temperature °C
Fig. 2. Differential thermogravimetric analysis of the thermal decomposition reaction of pure KC104 (curve a) and of KC104 catalysed with pure Cr203 (curve b) and doped with 5 tool.% Li+ (curve c), 5 tool.% Zr 4+ (curve d) and 5 mol.% U 6+ (curve e). TABLE 1 Decomposition temperature T d for pure and mixed KC104 as shown in the differential thermogravimetric plots of Fig. 2
System
Decomposition temperature (°C)
KCIO4 KCI04-10%Cr203 KCIO4-10%Cr203- 5mol.%Li + KC104-10%Cr203-5mol.%Zr 4+ KCIO4-10%Cr2Oa-5mol.%U 6+
590 465 450 435 405
Oz + L i 2 0 = 2 L i [ C r [ " + CrzO 3 + 4let"
(5)
w h e r e ZriCr]" a n d LifCr[" r e s p e c t i v e l y r e p r e s e n t t h e Zr 4+ a n d Li ÷ ions w h i c h r e p l a c e t h e Cr 3+ ions in t h e i r lattice positions. Such m e c h a n i s m s o f d o p i n g can be e x p r e s s e d in this m a n n e r if we c o n s i d e r t h a t t h e o x y g e n gas evolved can be c h e m i s o r b e d o v e r t h e Cr2Oa surface, t h u s c o n s u m i n g s o m e o f t h e electrons, w h i c h s a t u r a t e s t h e d e f e c t elect r o n s lel" c r e a t e d w h e n Zr 4÷ o r U 6+ w e r e used. This p h e n o m e n o n is well
121
-/,
b
-5
-
-
-¢
_J
-6
-7
--
--
I
I
I
1.5
2.0
2.,5 1/T X 10 3
K
Fig. 3. log a vs. 1 / T for pure Cr203 (curve a) and for Cr203 doped with Li+ (curve b), Zr4+ (curve c) and U 6+ (curve d).
TABLE 2 Oxidation power determined by the KI method for pure Cr203 and Cr203 doped with foreign ions calcined at 600 °C for 5 h Catalyst
Oxidation p o w e r (rag equivalent (g 02-) -1)
Pure Cr203 Cr203-5mol.%Li+ Cr203-5mol.%Zr 4+ Cr203-5mol.%U 6+
6.9 15.3 7.6 7.1
supported by determining the oxidation power o f the oxygen species as for NiO [17], as shown in Table 2. These results strongly support the explanation of why Cr203 doped with Zr 4+ has higher o values than Cr203 doped with U 6÷ ions. The contradictory large oxidation power o f Cr203 doped with Li+ ions compared with its conductance behaviour which is nearly similar to t h a t o f the pure oxide is mainly explained in that the holes created by the Li÷ ions need a higher
122
activation energy than the rest o f the samples to make their contribution to the conduction process (because of the high electronegativity of the Li÷ ions). To emphasize the role o f such mechanisms in the decomposition process, o values were measured as the decomposition reaction proceeded. The results are presented graphically in Fig. 4. It appears that 0 increases when the reaction temperature increases, reaching a m a x i m u m at 330 °C. As discussed before [12], this m a x i m u m is assigned to the phase transition of KC104. The increase in a with increase in temperature in this region is mainly due to the increase in the ionic mobility o f the KClO4 salt. With increases in the reaction temperature, other maxima were obtained, their locations depending on the additive type. These second maxima are caused by the release of electrons during the decomposition of the KC104 salt. It appears from Fig. 4 that the shift in the positions of the second maxima is in the order Cr20 a + U 6+ ~ Cr203 + Z r 4+ ~ Cr203 + Li÷. After complete thermal decomposition of catalysed KClO4 the o values are nearly identical (log o is around --4.3). As mentioned before [12], since Cr:O3 has p - n - t y p e semiconductive properties at high temperatures, the decomposition reaction will be catalysed by both electrons and holes. As seen before, the addition of foreign ions affects its semiconductive properties. The results of the activity measure-
-2
~4
Io -J
-6
-B
13
I
I 15
i
I 17
i
I 19
,
I 21 I / T X 104
K
I 23
K
Fig. 4. V a r i a t i o n in 0 w i t h 1/T for t h e t h e r m a l d e c o m p o s i t i o n r e a c t i o n o f KC104 catalysed with Cr20 a a n d d o p e d with Li + (curve a), Z r 4+ (curve b) a n d U 6+ (curve c) ions.
123
ments indicate that increasing the electron concentration by the addition o f U 6÷ or Zr 4+ to the Cr203 lattice will enhance the catalytic decomposition rate more than increasing the hole concentration by the addition of Li ÷ ions. Finally, it is o f interest to note here that the IR spectra o f all the solid products after the decomposition reaction correspond to the K2Cr207 salt. This confirms that the constant value of a at the end of the process is attributable to the lattice c o n d u c t i o n o f this salt.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
K.M. Abd El-Salaam and K. Hauffe, Ber. Bunsenges. Phys. Chem., 83 (1978) 811. K. Hauffe and H. Grunewald, Z. Phys. Chem., 198 (1951) 248. A. Zecchina, C. Morterra, G. Ghiotti and E. Borello, J. Phys. Chem., 73 (1969) 1292. Y. Komatsu,Bull. Chem. Soc. Jpn., 41 (1968) 167. A. E. T. Kuiper, J. Mederna and J. J. G. Van Bokhoren, J. Catal., 12 (1968) 19. H. Schafer and E. Buckler, Z. Naturforsch., TeilA, 23 (1968) 1685. W. A. F. Fischer and H. Dietrich, Z. Phys. Chem., 41 (1964) 205. F. N. Tavadzo, O. I. Mikadze, B. P. Buliya and V. K. Gilderman, Soobshch. Akad. Nauk Gruz. S.S.R., 102 (1981) 361. S. Rusiecki, Bull. Acad. Pol. Sci., Set. Sci. Chim., 29 (1981) 155. W. Kramarz and J. Zajecki, PoL J. Chem., 54 (1980) 2075. B. Heinrich and S. Helmut, Z. Anorg. Allg. Chem., 329 (1964) 18. A. A. Said, E. A. Hassan and K. M. Abd El-Salaam, Surf. Technol., 20 (1983) 131. J. Deren, J. Haber, A. Podgoreck and J. Burzyk, J. Catal., 2 (1963) 161. K. M. Abd El-Salaam and A. A. Said,Surf. Technol., 17 (1982) 199. G. A. E1-Shobaky, Th. EI-Nabarawy and T. M. Ghazy, Surf. TechnoL, 15 (1982) 153. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 1980, p. 1028. T. Uchijima, M. Takahashi and Y. Yoneda, J. Chem. Soc. Jpn., 40 (1967) 2767.
117
INFLUENCE OF ADDITIVES TO CHROMIUM OXIDE CATALYSTS FOR THE THERMAL DECOMPOSITION OF KClO4
E. A. HASSAN, A. A. SAID and K. M. ABD EL-SALAAM Chemistry Department, Assiut University, Assiut (Egypt)
(Received June 22, 1983)
Summary The effect of doping on the catalytic behaviour of Cr203 was studied using the thermal decomposition of KCIO4 as a test reaction. The doping of the catalysts was effected using 5 mol.% Li÷, Zr 4÷ or U 6÷ ions and calcination at 600 °C for 5 h. Studies were carried out using thermogravimetric analysis and electrical conductivity measurements. The results showed that Cr203 was more effective in enhancing the decomposition process when it was doped with U 6÷ ions than when it was doped with Li÷ or Zr 4÷ ions. Mechanisms of doping were discussed and the role of defects in the catalytic reaction was interpreted.
1. Introduction Several interesting discussions have concerned the relationship between the electronic structure o f solids and their catalytic activity [1, 2]. Several researchers have been concerned with the surface structure of solid chromium oxide with particular reference to the identification of the active sites responsible for the catalytic activity [3 - 6 ] . Fischer and Dietrich [7] studied the electrical behaviour o f Cr203 and its dependence on the temperature and partial pressure of oxygen. It was found that Cr203 was an electron~leficient conducting solid. This result was in accordance with that of Tavadzo et al. [8], who indicated that Cr203 is a p-type semiconductor and at high temperatures the structure becomes disordered, possibly as a result of Frenkel defect formation. Incorporation of CdO into Cr203 was examined [9] at 873 K and it was found using the electronic conduction m e t h o d that oxygen vacancies are the predominant defect. Addition of a monovalent cation to Cr203 showed an increase in the conductivity and had a great influence on its activity [10]. The activity of Cr203 as a dehydrogenation catalyst for 2-propanol was examined where a reversible p - n transition was observed [11]. The dehydrogenation dominated with increasing n-type character and increasing conductivity. 0376~4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
118 According to work reported in the literature and our previous studies [12] it is o f interest to study the effect o f additives such as Li +, Zr 4÷ or U 6÷ ions to the Cr203 lattice structure on t he catalytic action o f Cr203 for the thermal d eco mp o s it i on o f KC104 as a test reaction.
2. Experimental details Chromium oxide catalysts d o p e d with 5 mol.% Li ÷, Zr 4+ or U 6÷ ions were prepared by mixing calculated amounts o f LiOH, Zr(NO3)3.3H20 or UO2(NO3)2.6H20 and Cr(NO3)3.3H20 in a porcelain dish using d o u b l y distilled water. The mixtures were stirred well, evaporated over a water bath to dryness and then dried in an oven at 110 °C to constant weight. The catalysts were calcined in a muffle furnace in an air stream at 600 °C for 5 h. The thermal decomposition reaction o f AnalaR KC104 was used for monitoring the doping effect on the catalytic behaviour o f Cr203. These studies were carried o u t using a gasometric technique [ 13 ]. The electrical conductivity measurements were carried o u t as previously discussed [14]. IR spectra were taken for the final d e c o m p o s i t i o n products using a P e r k i n - E l m e r s pe c t r om e t e r model 599B with t he KBr disc technique. The oxidation powers o f t h e catalysts were determined using t he KI m e t h o d [15].
3. Results and discussion Thermograms o f pure and catalysed KC104 decom posi t i on are shown in Fig. 1. Curve a represents the thermal de c omposi t i on o f pure KC104 at a pressure o f 1 atm. The decomposition o f KC104 catalysed with 10% Cr203 is represented by curves b - e. The general t r e nd is for the catalyst to cause an e n h a n c e m e n t in the decom pos i t i on reaction to a certain degree, depending on t h e catalyst preparation. Also, it appears t h at the most effective doping o f the Cr203 catalyst is by U 6÷ ions. The exact differences in the decomposition temperatures can be seen when differential thermogravimetric plots are constructed as shown in Fig. 2. A s um m a r y o f the results is given in Table 1. Such a great difference in t he Td values m a y be understood by studying t h e mechanism o f doping o f t he Cr203 structure. Figure 3 shows t he variation in log a against 1/T f or pure and doped Cr203 catalysts. The results show th at when Cr203 is doped with U 6+ ions it has a smaller conductivity value at any t e m p e r a t u r e than when it is d o p e d with Li ÷ or Zr 4+. It is well k n o w n that Cr203 is a p-type s e m i conduct or [8]; its c o n d u c t a n c e charge carriers are holes according to 3 1 - - 0 2 = [Cr]"' + 3[el" + --Cr203 4 2
(1)
where [Cr[ '" is a cation vacancy and [el" is a defect electron. The addition of
119
9ok to
O-
E
70
123
50
3O
~0
200
300
L.O0
500 Temp.°C
Fig. 1. Thermogravimetrie analysis of the thermal decomposition of KCIO4 (curve a) and of KCIO4 catalysed with pure 10% Cr203 (curve b) and doped with 5 tool.% Li+ (curve c), 5 tool.% Zr4+ (curve d) and 5 tool.% U6+ (curve e).
U 6+ ions consequently causes a decrease in the n u m b e r o f these charge carriers according to the following mechanism: 2Jel" + 2UO2 = 2UICri" + Cr203 + iOl'"
(2)
6[el" + 2UO3 = 2 U [ C r [ " " + Cr203 + 3101""
(3)
where U 6+ ions exist in the oxide as t he U3Os structure [16] and diffuse into t h e Cr203 solid lattice according to mechanisms (2) and (3). UHCr[" and U [ C r I " " represent U 4+ and U 6+ which replace the Cr 3÷ in their normal lattice site positions and IO]'" is an o x y g e n anion vacancy. According to these mechanisms a decrease in the ie[" concent rat i on is predicted, which eventually leads to a r e duc t i on in o as observed in Fig. 3. Adopting such a role for the doping effect, t hen Zr 4÷ ions can be i ncorporat ed in the following way: 21el" + 2ZrO2 = 2ZrlCri" + Cr203 + ]O['"
(4)
Such a mechanism will lead to a decrease in the hole c o n c e n t r a t i o n and consequently to a reduction in a compared with t he conductivity o f pure Cr20 3 which is n o t f ound experimentally. Also, with Li + ions, doping could be effected in the following way:
120
I
1e
I
I
t
11 II
II
iI
d I
, I
b
~1 / .~Ii i~,ll
,
o
I, !t,
IL i,!, II
l
I' , 300
\
: ;i,/I I
[I
I!Jl
I
~ill
I
/
~
/
' i'a I
/
/* .ff /.00
500
600
Temperature °C
Fig. 2. Differential thermogravimetric analysis of the thermal decomposition reaction of pure KC104 (curve a) and of KC104 catalysed with pure Cr203 (curve b) and doped with 5 tool.% Li+ (curve c), 5 tool.% Zr 4+ (curve d) and 5 mol.% U 6+ (curve e). TABLE 1 Decomposition temperature T d for pure and mixed KC104 as shown in the differential thermogravimetric plots of Fig. 2
System
Decomposition temperature (°C)
KCIO4 KCI04-10%Cr203 KCIO4-10%Cr203- 5mol.%Li + KC104-10%Cr203-5mol.%Zr 4+ KCIO4-10%Cr2Oa-5mol.%U 6+
590 465 450 435 405
Oz + L i 2 0 = 2 L i [ C r [ " + CrzO 3 + 4let"
(5)
w h e r e ZriCr]" a n d LifCr[" r e s p e c t i v e l y r e p r e s e n t t h e Zr 4+ a n d Li ÷ ions w h i c h r e p l a c e t h e Cr 3+ ions in t h e i r lattice positions. Such m e c h a n i s m s o f d o p i n g can be e x p r e s s e d in this m a n n e r if we c o n s i d e r t h a t t h e o x y g e n gas evolved can be c h e m i s o r b e d o v e r t h e Cr2Oa surface, t h u s c o n s u m i n g s o m e o f t h e electrons, w h i c h s a t u r a t e s t h e d e f e c t elect r o n s lel" c r e a t e d w h e n Zr 4÷ o r U 6+ w e r e used. This p h e n o m e n o n is well
121
-/,
b
-5
-
-
-¢
_J
-6
-7
--
--
I
I
I
1.5
2.0
2.,5 1/T X 10 3
K
Fig. 3. log a vs. 1 / T for pure Cr203 (curve a) and for Cr203 doped with Li+ (curve b), Zr4+ (curve c) and U 6+ (curve d).
TABLE 2 Oxidation power determined by the KI method for pure Cr203 and Cr203 doped with foreign ions calcined at 600 °C for 5 h Catalyst
Oxidation p o w e r (rag equivalent (g 02-) -1)
Pure Cr203 Cr203-5mol.%Li+ Cr203-5mol.%Zr 4+ Cr203-5mol.%U 6+
6.9 15.3 7.6 7.1
supported by determining the oxidation power o f the oxygen species as for NiO [17], as shown in Table 2. These results strongly support the explanation of why Cr203 doped with Zr 4+ has higher o values than Cr203 doped with U 6÷ ions. The contradictory large oxidation power o f Cr203 doped with Li+ ions compared with its conductance behaviour which is nearly similar to t h a t o f the pure oxide is mainly explained in that the holes created by the Li÷ ions need a higher
122
activation energy than the rest o f the samples to make their contribution to the conduction process (because of the high electronegativity of the Li÷ ions). To emphasize the role o f such mechanisms in the decomposition process, o values were measured as the decomposition reaction proceeded. The results are presented graphically in Fig. 4. It appears that 0 increases when the reaction temperature increases, reaching a m a x i m u m at 330 °C. As discussed before [12], this m a x i m u m is assigned to the phase transition of KC104. The increase in a with increase in temperature in this region is mainly due to the increase in the ionic mobility o f the KClO4 salt. With increases in the reaction temperature, other maxima were obtained, their locations depending on the additive type. These second maxima are caused by the release of electrons during the decomposition of the KC104 salt. It appears from Fig. 4 that the shift in the positions of the second maxima is in the order Cr20 a + U 6+ ~ Cr203 + Z r 4+ ~ Cr203 + Li÷. After complete thermal decomposition of catalysed KClO4 the o values are nearly identical (log o is around --4.3). As mentioned before [12], since Cr:O3 has p - n - t y p e semiconductive properties at high temperatures, the decomposition reaction will be catalysed by both electrons and holes. As seen before, the addition of foreign ions affects its semiconductive properties. The results of the activity measure-
-2
~4
Io -J
-6
-B
13
I
I 15
i
I 17
i
I 19
,
I 21 I / T X 104
K
I 23
K
Fig. 4. V a r i a t i o n in 0 w i t h 1/T for t h e t h e r m a l d e c o m p o s i t i o n r e a c t i o n o f KC104 catalysed with Cr20 a a n d d o p e d with Li + (curve a), Z r 4+ (curve b) a n d U 6+ (curve c) ions.
123
ments indicate that increasing the electron concentration by the addition o f U 6÷ or Zr 4+ to the Cr203 lattice will enhance the catalytic decomposition rate more than increasing the hole concentration by the addition of Li ÷ ions. Finally, it is o f interest to note here that the IR spectra o f all the solid products after the decomposition reaction correspond to the K2Cr207 salt. This confirms that the constant value of a at the end of the process is attributable to the lattice c o n d u c t i o n o f this salt.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
K.M. Abd El-Salaam and K. Hauffe, Ber. Bunsenges. Phys. Chem., 83 (1978) 811. K. Hauffe and H. Grunewald, Z. Phys. Chem., 198 (1951) 248. A. Zecchina, C. Morterra, G. Ghiotti and E. Borello, J. Phys. Chem., 73 (1969) 1292. Y. Komatsu,Bull. Chem. Soc. Jpn., 41 (1968) 167. A. E. T. Kuiper, J. Mederna and J. J. G. Van Bokhoren, J. Catal., 12 (1968) 19. H. Schafer and E. Buckler, Z. Naturforsch., TeilA, 23 (1968) 1685. W. A. F. Fischer and H. Dietrich, Z. Phys. Chem., 41 (1964) 205. F. N. Tavadzo, O. I. Mikadze, B. P. Buliya and V. K. Gilderman, Soobshch. Akad. Nauk Gruz. S.S.R., 102 (1981) 361. S. Rusiecki, Bull. Acad. Pol. Sci., Set. Sci. Chim., 29 (1981) 155. W. Kramarz and J. Zajecki, PoL J. Chem., 54 (1980) 2075. B. Heinrich and S. Helmut, Z. Anorg. Allg. Chem., 329 (1964) 18. A. A. Said, E. A. Hassan and K. M. Abd El-Salaam, Surf. Technol., 20 (1983) 131. J. Deren, J. Haber, A. Podgoreck and J. Burzyk, J. Catal., 2 (1963) 161. K. M. Abd El-Salaam and A. A. Said,Surf. Technol., 17 (1982) 199. G. A. E1-Shobaky, Th. EI-Nabarawy and T. M. Ghazy, Surf. TechnoL, 15 (1982) 153. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 1980, p. 1028. T. Uchijima, M. Takahashi and Y. Yoneda, J. Chem. Soc. Jpn., 40 (1967) 2767.